Axial piston pumps are well known in the art. A typical axial piston pump comprises of a cylinder block on which a number of cylinder bores are made and a piston assembly is disposed in a sliding manner in each of the cylinder bores. The piston assemblies connect to a swashplate, which translates a rotating motion to the reciprocating motion of the pistons. During operation, the pistons reciprocate in the cylinder bores of the cylinder block either by rotating cylinder block itself while the swashplate standing still or by rotating the swashplate while the cylinder block is standing still. In either model, rotating cylinder block or rotating swashplate, the clearance between the cylinder wall and a reciprocating piston is critical to the performance of the axial piston pump because the leakage between the cylinder wall and reciprocating piston, which is called internal leakage, is one of the greatest factors contributing to fatal power loss of the axial piston pump.
The typical axial piston pumps are designed and manufactured for the operating temperature range of −30° C. to +150° C. The alloy for the cylinder block is usually copper based brass family for the bearing functionality and the alloy for piston is usually chromium based hard steel for the higher durability. Using two different alloys leads to the two parts having different thermal expansion rates along the atmospheric and internal temperature changes. It in turn causes the expansion and contraction of the clearances between the cylinder walls and the pistons. Stuck cylinders under high temperature and severe leakages under low temperature are major problems. Therefore, the optimum clearance is one that is large enough to avoid the stuck cylinder condition under high temperature, but small enough to prevent sever leakages under low temperature. Traditionally, the achievement of optimum clearance relies solely on machining and finishing accuracy of the piston and cylinder bore during manufacturing. However, the wear and tear of the cylinder and piston over time, thus deviation from the optimum parameter, is unavoidable.
The competing criteria imposed by the clearance size and thermal expansion and contraction characteristics also pose difficult manufacturing challenges including a narrow selection of cylinder block and piston materials and applicable heat treatment processes.
The presently claimed invention is directed to overcoming the aforementioned problems by providing an axial piston pump with pistons having metallic sealing rings.